Fungi impact human existence as opportunistic pathogens and as important industrial biocatalysts.The long term goals of this research are to provide new information on fungal cell division. Cytokinesis, is a fundamental growth process in all cells and involves the coordination of cell growth and the nuclear division cycle. Web have chosen to analyze this process in the filamentous fungus, Aspergillus nidulans. This genetically tractable eukaryotic microbe has a rich history of molecular genetic research and is a close relative of many medically and industrially important microorganisms. In A. nidulans cytokinesis occurs by the formation of crosswalls termed septa. We have shown that septum formation is an actin dependent process, temporally coordinated with mitosis. We have identified and characterized ten genes, designated sepA, B, C, D, E, G, H, I, J and K carrying temperature sensitive (ts) mutations that block cytokinesis. These cytokinesis mutants sort into three phenotypic classes. Class 1 mutants (B, C, E, I and J) appear to be involved in early steps in signalling septum formation. Class 2 mutants (A and K) may be defective in actin dependent processes such as localized cell wall growth. Class 3 mutants (D,G and H) appear to be blocked at later stages of septum formation. Together with data from physiological experiments, we have formulated a genetic model for septum formation. The sepA and sepB genes have been cloned. Finally we have identified three A. nidulans homologs of the Saccharomyces cerevisiae bud-neck filament genes encoded by the CDC3, 10, 11 and 12 genes. We have designated these loci ANFl, 2and 3. The specific goals of this proposal are to determine the specific functions of several sep genes representing the three phenotypic classes: specifically sepA, B, C, D, E and K genes. This will be done by cloning the genes and characterizing the protein products they encode with regard to expression, cellular localization and similarities to other known proteins. A series of genetic experiments will be used to assess sep gene function. These include the construction of double mutants to assess interactions between sep gene products. We have devised genetic screens to identify additional genes involved in the cellularization process. Mutants will be sought that lack septa, mis-regulate the timing of septum formation, or form precocious numbers of septa. Mutations will be constructed in the ANF genes to assess their role in cytokinesis. Finally probes developed from the above research will be used to more precisely define the phenotypic defect in the sep mutants as well as to define the temporal order of events for cytokinesis in A. nidulans. The results from our research will provide new information on fungal growth and differentiation. Due to apparent similarities to cytokinesis in animal cells, a genetic and molecular analysis of septation may elucidate the roles of regulatory or structural proteins common to both processes.